1. Field of the Invention
The present invention relates to an optical receiver, in particular, the invention relates to an optical receiver having a variable bias source for a photodiode (hereafter denoted as PD).
2. Related Prior Arts
A publicly available document of, “100 GbE-Optical LAN Technology”, published by IEEE Application and practice, December 2007, p.p 12-19 has introduced the specification of an optical transceiver applicable to the 100 Gb-EATHERNET (Trademark). Such a transceiver may realize the transmission speed of 100 Gbs by multiplexing four optical beams each having a specific wavelength different from others and a speed of 25 Gbps. The transceiver is necessary to install four transmitter optical subassemblies (hereafter denoted as TOSA) and four receiver optical subassemblies (hereafter denoted as ROSA).
The standard introduced therein has taken a condition into account where the TOSA may be implemented with a directly modulated laser diode (hereafter denoted as LD) in addition to an externally modulated LD operated as a type of so-called electro-absorption modulator. It becomes hard to operate the directly modulated LD in a speed of about 25 Gbps. An increased bias current may possibly operate the directly modulated LD but the optical output power emitted from the TOSA inevitably becomes large.
The standard also defines the condition for the ROSA, that is, the ROSA is necessary to recover an optical signal received thereby even when the ROSA receives an overloaded input, for instance, a magnitude of the input optical power exceeding 4.5 dBm. Assuming the transmission distance of about 10 km, the optical input power to the ROSA generally becomes about −6.0 dBm, which is far less than the overloaded optical input power. Then, when the PD implemented in the ROSA is set to receive such optical signal with power of about −6.0 dBm, the PD is also necessary to show enough tolerance for the overload optical input power.
However, the PD in the quality of the output electrical signal therefrom generally depends on the received input power. For instance, a jitter appearing in the electrical output of a PD often increases as the optical input power increase. The jitter causes a faint degradation in the transmission quality, for instance, the bit error rate, when the transmission speed is relatively slower; but the jitter shows fatal results in the communication system when the transmission speed reaches or exceeds 25 Gbps.
The invention of the present application relates to an optical receiver that comprises a PD, voltage source and a current detector. The PD generates a photocurrent depending on a bias applied thereto. The voltage source provides the bias. The current detector detects the photocurrent. A feature of the present optical receiver is that the current detector and the voltage source forms a positive feedback loop from the photocurrent to the bias, namely, the bias being increased as the increase of the photocurrent, but the loop gain of the feedback loop is set to be less than unity.